This invention relates to the making of tubes from synthetic thermoplastic material.
More particularly, the invention relates to an apparatus for making such tubes, especially tubes which have corrugated walls.
It is known, e.g., from German patent No. 1,753,625, to extrude a tubular parison of thermoplastic material into a travelling mold and therein to convert it -- by internal overpressure or by external suction in the travelling mold -- into a corrugated tube. The apparatus has two strings of mold sections; the mold sections of each string travel in an endless path which includes a straight path portion. While travelling in this straight path portion each mold section of one string cooperates with a section of the other mold section to form therewith a travelling mold. The inner surfaces of the mold sections are formed with ribs or the like so that, when the thermoplastic tubular parison in the travelling mold is expanded into conformance with these inner surfaces, it will be formed with corrugations.
At the beginning and at the end of the straight path portion the mold sections of the respective strings must be respectively guided towards and away from one another. This cannot be done by making them travel in arcuate paths because it would lead to damage to the mold sections, due to the fact that the radius on which the trailing ends of the mold sections would travel relative to the respective arcuate path, is greater than the radius on which the centers of these mold sections travel.
The German patent seeks to overcome the problem by providing each mold section with an inclined face which, when the mold section enters the region of the beginning of the straight path portion, slide along correspondingly inclined guide faces of the apparatus; after leaving the substantially semi-circular deflecting region the segments therefore slide along the inclined guide faces until they enter the straight path portion. During this travel each segment is shifted parallel to itself. An analogous arrangement is provided in the region of the end of the straight path portion.
This prior-art construction has been used satisfactorily for years. However, there is a demand for this type of tube to be furnished with even larger diameters, and the larger the tube diameter the more complicated becomes the construction of a machine using the aforementioned mold-section guiding arrangement. Also, the amount of energy required to move the mold sections become greater the larger the tube diameter because the friction of the correspondingly larger -- and heavier -- mold sections on the machine guide faces must be overcome. In addition, the machine guide faces and the mold-section faces are subject to substantial wear, so that the amount of play between the mold sections and the guide faces increases steadily and finally exceeds permissible limits.